The present invention relates to methods for making high-performance superconducting materials. In particular, the present invention relates to a method for making films of biaxially textured YBa2Cu3O7 (xe2x80x9cYBCOxe2x80x9d) on flexible substrates.
The recent discovery of a broad class of materials such as YBCO with superconducting transition temperatures exceeding the boiling point of liquid nitrogen (77 K) has resulted in widespread interest in developing methods for making these materials and finding new applications for them. YBCO is useful for the fabrication of high frequency filters, SQUIDS and other electronic devices. In addition it has promise as a conductor for use in electric motors, electrical cable, transformers, fault current limiters and various electrical utility devices. One of the most promising applications of high critical temperature superconductors is for use in the fabrication of high field magnets. The fabrication of magnets requires the availability of flexible superconductors, especially YBCO superconducting tapes. YBCO superconducting tapes are generally fabricated by growing an YBCO film on a flexible substrate. The most desirable YBCO films for applications in flexible tapes have a thickness of about 1 micron or greater. The YBCO films must be relatively free of impurities and various structural defects, such as high angle grain boundaries, so that they are capable of carrying high critical current densities, Jc. The flexible superconducting tapes must also be fabricated economically.
Many different techniques for fabricating YBCO films are described in the scientific literature. These methods include, but are not limited to, RF sputtering, DC sputtering, magnetron sputtering, thermal evaporation, electron beam evaporation, pulsed laser deposition (PLD), spin coating, dip coating, metal organic deposition (MOD) and metal organic chemical vapor deposition (MOCVD).
One of the methods for making textured crystalline YBCO is the so-called xe2x80x9cBaF2 post annealing processxe2x80x9d. This process consists of first preparing a precursor film consisting of a mixture of BaF2, Y and Cu deposited on a supporting substrate. To transform the precursor film into a crystalline YBCO film, the precursor film is heat-treated in a humidified, low oxygen partial pressure atmosphere. The BaF2 process as currently practiced heat-treats the films at atmospheric pressure.
Another method for making crystalline YBCO films on a substrate involves coating a substrate with a solution and thermally processing the resulting composite. This technique is referred to as metal organic deposition (MOD). The first step in the MOD process is to prepare a liquid solution, for example a mixture of Y, Ba, and Cu trifluoroacetates in methanol. Substrates are coated with the solution and allowed to dry forming a precursor film. The organic components of the precursor film are xe2x80x98burned offxe2x80x99 at about 400xc2x0 C. The film-substrate composite is next heat-treated in a humidified, low oxygen partial pressure atmosphere converting the precursor film to a crystalline YBCO film. The MOD technique as currently practiced heat-treats the films at atmospheric pressure.
Both of these techniques, as described in the literature, require heat-treating the precursor films at atmospheric pressure, at temperatures above about 600xc2x0 C. Both techniques also incorporate fluorine (F) in the precursor film.
Unfortunately both of these techniques for making crystalline YBCO films have drawbacks. One drawback is that the YBCO film growth rate is low. In experiments performed at Brookhaven National Laboratory (BNL), YBCO films 5 microns thick were grown at growth rates of about 0.7 Angstroms per second. These films were synthesized using an atmospheric pressure heat-treating gas composed of 25 Torr water vapor, 100 milliTorr oxygen and balance nitrogen. The heat-treating temperature was 735xc2x0 C. and the total gas flow was 100 standard cubic centimeters per minute (sccm). In order to achieve higher growth rates it was necessary to substantially increase the water vapor pressure. YBCO films 5 microns thick were grown at BNL at rates of up to 2.5 Angstroms per second using a heat-treating temperature of 735xc2x0 C., 100 milliTorr of oxygen and a total gas flow of 100 sccm. These YBCO films were c-axis oriented and had high Jc greater than 1xc3x97105 Ampere per square centimeter (0.1 MA/cm2), but extremely high water vapor partial pressures, about 150 Torr, were required during the heat-treating step. This work is described in the BNL article by Solovyov et al. xe2x80x9cHigh Rate Deposition of 5 um Thick YBa2Cu3O7 Films Using the BaF2 Ex-Situ Post Annealing Processxe2x80x9d, IEEE Transactions on Applied Superconductivity, 9 (2), June 1999, incorporated by reference herein. Such extremely high water vapor pressures can have a corrosive effect on the supporting substrate, processing furnace 10 and possibly the YBCO film itself. In addition the equipment required for delivering the humidified gas to the heat-treating furnace becomes more complex and costly. Finally, it is difficult to grow large area films with uniform properties using atmospheric pressure heat-treating.
An experimental vacuum-processing apparatus was constructed for heat-treating precursor films at sub-atmospheric pressures over a temperature range of room temperature to 1000xc2x0 C. The water vapor, oxygen and carrier gas partial pressures present in the apparatus could be independently controlled during heat-treatment. It was discovered that during heat-treatment the water vapor partial pressure can be decreased by a factor of more than 1000 and the total processing gas pressure decreased by a factor of over a 100 and films with high Jc can be grown at rates exceeding 5 Angstroms per second (xc3x85/sec.). It was also discovered that the growth rate of crystalline YBCO films was inversely proportional to the to the total gas pressure, PT, in the vacuum-processing apparatus. Decreasing the total gas pressure increased the YBCO growth rate.
An additional benefit is a decrease in the residence time of the gasses when heat-treating at sub-atmospheric pressures while using the same gas flow as in an atmospheric pressure processing chamber. A decrease in residence time allows impurity gasses and gaseous by-products to be swept out of the sub-atmospheric heat-treating chamber much more quickly. This results in YBCO films of higher purity as compared to YBCO films synthesized at atmospheric pressure. In addition the quantity of oxygen and carrier gas can be substantially reduced using sub-atmospheric heat-treating as compared to atmospheric heat-treating making sub-atmospheric heat-treating more economical.
U.S. Pat. No. 5,306,698 to Ahn et al. discloses a method for producing a high transition temperature superconductor of Tl2Ca2Ba2Cu3 oxide from a Tl2Ca2Ba2Cu3 cation composition. Ahn et al. use an annealing step in a reduced oxygen atmosphere to convert compounds containing thallium, calcium, barium and copper to a Tl-2223 superconducting phase or to convert an oxide having the nominal composition Tl2Ca2Ba2Cu3Ox to a crystalline Tl-2223 phase.
U.S. Pat. No. 5,308,800 to Wehrle et al. discloses an apparatus and method for fabricating textured pseudo-single crystal bulk superconducting materials. Wehrle et al. produce these bulk superconducting materials by melt-texturing techniques using a steep temperature gradient furnace at elevated pressures of from 10 to 20 atmospheres. Wehrle et al. disclose that highly textured superconductor materials have been prepared which exhibit superconductive properties at higher temperatures than previously achievable.
U.S. Pat. No. 5,661,114 to Otto et al. discloses a method of preparing BSCCO oxide superconductor articles using a low pressure, low temperature annealing process to increase the critical current density. Otto et al. disclose that the process conditions are adjusted so that the article is partially melted and a liquid phase co-exists with the desired oxide superconductor phase. Otto et al. also disclose that the oxide superconductor articles can be formed from the YBCO-123 family.
U.S. Pat. No. 5,741,377 to Goyal et al. discloses a method for preparing a biaxially textured alloy article by depositing an alloying agent onto a substrate to form a laminate and heating the laminate. Goyal et al. also disclose c-axis oriented, biaxially textured, superconducting YBCO films.
U.S. Pat. No. 5,850,098 to Butler et al. discloses a thermal detector that includes a transducer layer of amorphous semiconducting yttrium barium copper oxide. Butler et al. also disclose that thick layers of YBa2Cu3O7 can be annealed at 500xc2x0 C. in a vacuum to crystallize the film.
U.S. Pat. No. 5,854,587 to Horwitz et al. discloses rare earth manganite thin films preferably formed by pulsed laser deposition on a Si3N4 substrate in a vacuum chamber with a buffer layer of CeO2 to help direct the manganite material to grow in the correct phase and optimum crystallographic orientation followed by annealing at a temperature in the range of 500xc2x0 to 900xc2x0 C. in oxygen or vacuum. However, Horwitz et al. only disclose manganite film structures.
U.S. Pat. No. 4,959,346 to Mogro-Campero et al. is directed to a composite comprised of Yxe2x80x94Baxe2x80x94Cuxe2x80x94O superconductive film having a zero resistance transition temperature of at least 38 K. However, unlike the present invention, the precursor film of Mogro-Campero et al. is annealed at atmospheric pressure and not sub-atmospheric pressure.
U.S. Pat. No. 5,856,277 to Chen et al. discloses a method of manufacture of a textured layer of a high temperature superconductor, such as rare earth superconductors and a YBa2Cu3O7-x(123-YBCO) superconductor on a substrate using a melt texturing process. Chen et al. also disclose a method that can be used to produce a film 0.1-1 xcexcm thick by heating the films to temperatures of 950xc2x0 to 990xc2x0 C. at sub-atmospheric pressures.
U.S. Pat. No. 6,172,009 to Smith et. al. discloses a metal oxyfluoride on a substrate. The metal oxyfluorides of Smith et. al. are ternary compounds of a metal bound to oxygen and fluorine, e.g. MOxFy or a mixture of MOxFy and metal oxides and metal fluorides e.g. MOx and MFy. Smith et. al. does not disclose the use of sub-atmospheric pressures nor do they teach the importance of the total absolute pressure of the processing gas, PT.
The present invention overcomes the problems experienced by processes currently in use by carrying out the heat treatment of precursor films at sub-atmospheric pressures.
The present invention is a method of forming films of crystalline YBa2Cu3O7 that includes forming a precursor film of barium fluoride (BaF2), yttrium (Y) and copper (Cu) on a substrate that preferably includes SrTiO3 or CeO2. The precursor film is heat-treated at a temperature above about 500xc2x0 C. and most preferably to about 735xc2x0 C. in the presence of oxygen, water vapor and optionally a carrier gas, preferably nitrogen, at sub-atmospheric pressure to form a crystalline YBa2Cu3O7-xcex4 structure. The partial pressure of oxygen during the heat-treating of the precursor film is about 100 milliTorr. The heat-treating is accomplished over a period of from about 30 minutes to about 14 hours depending on the thickness of the starting precursor film and the desired growth rate. The crystalline structure is then annealed in the presence of oxygen at a temperature of from about 400xc2x0 C. to about 600xc2x0 C. to form a fully oxygenated crystalline YBa2Cu3O7 film.
During the heat-treating step, the YBa2Cu3O7 film grows at a rate of from about 1 to about 20 Angstroms per second as measured by the conductivity of the film. The thickness of the YBa2Cu3O7 films produced by this method is from about 0.05 to about 5 microns, preferably from about 0.2 to about 3 microns.
The present invention provides high quality YBa2Cu3O7 films that are more uniform and can be grown at relatively high growth rates using substantially reduced water vapor pressure and substantially reduced total pressure than films made by the methods that are currently being used.